Printers, such as ink-jet printers, include mechanical assemblies to affect the transfer of ink onto a moving sheet of paper. These mechanical assemblies can include multiple ink-jets or other ink transfer mechanics that need to work correctly in unison in order to correctly print images on the paper.
As an example, a color ink-jet printer typically includes both a black ink printhead and plumbing assembly and a colored ink printhead and plumbing assembly. Colored or even black-and-white printers may also include multiple print-heads to affect denser dots-per-inch (DPI) and/or faster printing capability. As is known in the art, the term “PEL” corresponds to the distance between neighboring dots (e.g., the distance of a PEL can be determined by taking the inverse of the DPI of the printer). In many cases the DPI of the printer may be different in each direction, having one DPI along the direction of the movement of the paper and a second DPI in an orthogonal direction between the nozzles of the printheads.
Whenever multiple print sources (e.g., multiple ink-jets, multiple print-heads, etc.) are incorporated into a single printer, the “alignment” of these image sources, in time and/or space, becomes an important determinant of the quality of the overall printed image.
For example, FIGS. 1a and 1b depict a problem when two different print sources are temporally misaligned (i.e., misaligned in time). In the example of FIG. 1a-b, two print sources are attempting to print a straight, vertical line 104 while a piece of paper 105 is moving vertically in the printable range of the print sources. Here, for simplicity, the print source used to print features 101 and 103 can be assumed to be of a first type (e.g., a black ink-jet assembly) while the print source used to print feature 102 is assumed to be of a second, different type (e.g., a colored ink-jet assembly).
To form the printed line, as the paper is moving in the print area of the two print sources, the first print source (e.g., the black ink jet) will first print feature 101. Then, the first print source will stop printing and the second print source (e.g., the color ink jet) will print feature 102. Then, the second print source will stop printing and the first print source will begin printing feature 103. Ink jet printers often operate on a drop wise basis. The ejection of droplets or “jetting”, in some cases, drops different volumes to form the printed PELS on paper. The position of PELs along the direction of web movement is controlled by the timing of drop jetting while the location of the PELS perpendicular to the web movement direction is controlled by the spatial placement of jetting nozzles.
Given that the ideal arrangement of these features creates a “dashed” line with centered/equi-distant dashes having alternating color and black “dashes” the printed line should not have exhibit any overlap of the features 101, 102, 103 that were printed by the different print sources. To correctly draw such a line in the direction that the paper is moving, the different print sources need to synchronously jet drops of ink at the correct moment in time. Specifically, the source that prints feature 101 needs to stop jetting PELs approximately when the source that prints the PELs of feature 102 starts to jet, to produce properly positioned segments of the dashed line. Likewise, the source that prints feature 102 needs to stop jetting PELS approximately when source that prints feature 103 starts to jet PELs, to produce properly positioned segments of the lower portion of the dashed line.
Because the print sources are electromechanical and/or fluidic machines, there can be variability between the jetting of ink drops that produces PEL misplacements on the printed output even if the timing of the electronic signal that triggers the jetting is very consistent. For example in the case of an ink-jet, the various tolerances of printhead nozzles locations, pressure, spacing etc. all lend themselves to drop misplacements relative to an ideal uniformly spaced printer grid.
As such, the printed line may appear as in FIG. 1b where the feature 102 undesirably overlaps with feature 101. That is, the source that printed feature 102 jetted drops too soon and/or the source that printed feature 101 jetted drops too late.
Moreover, whereas FIGS. 1a-b pertain to temporal mis-alignment between two different colors, FIG. 2 pertains to an example of temporal mis-alignment of interleaved printheads having the same color. Inset 210 shows an interleaved print head approach that attempts to increase the resolution of the printed image by deliberately offsetting the alignment of two print heads 201, 202. Here, for example, if both print heads 201, 202 correspond to a 600 DPI print-head, a printed image resolution of 1200 DPI can be achieved by off-setting the lateral alignment between them by one half of the PEL distance 203 of each print head.
In a typical interleaved print-head solution, one of the print heads is used to print odd number columns and the other print head is used to print even numbered columns. Here, the entire printed image is viewed as a matrix of rows and columns where each element of the matrix corresponds to a printed image “dot” (which is also frequently referred to as a PEL). Thus, “left” print head 201 is configured to print the odd numbered columns and “right” print head 202 is configured to print the even numbered columns.
FIG. 2 demonstrates that a printing error like the printing error of FIG. 1b can result if the vertical spacing 204 between the print heads is misaligned. Here, an attempt is made to print a vertical “line” where the line consists of alternating odd and even columned dashes. Because of the intentional lateral alignment 203 between the dual print-heads, note that features 205, 207 printed with print head 201 in an odd column are offset and not horizontally aligned with the feature 206 printed with print head 202 in an even column (the pattern is printed with the first print head 201 printing feature 205, then the second print head 202 printing feature 206, then the first print head printing feature 207).
As observed in FIG. 2, if the vertical spacing 204 between the print heads 201, 202 is smaller than the nominal value and the printing between 205, 206 and 207 is nominal, the even column printed feature 206 can overlap or nearly overlap with the odd column printed image data.
The printing error of FIG. 2 results from a temporal alignment between the two print heads 201, 202 as discussed above with respect to FIGS. 1a and 1b, which must account for variations in spacing 204 between the interleaved printheads.
FIG. 3 shows a prior art test pattern that can be used to detect either temporal or mechanical mis-alignments in a printer having interleaved multiple print sources. The web movement direction is vertical and therefore the timing between the jetting of each printhead influences the vertical position of the lines. Each line is dashed so that a line is printed only by the nozzles of its respective interleaved printhead. Generally, the position of horizontal middle lines 301 is observed between an arrangement of horizontal outer line pairs 302. Misalignments are detected by observing when a horizontal middle line 301 becomes too close to a horizontal outer line 302. Unfortunately determination of the amount of misalignment, and therefore the necessary alignment compensation, cannot be easily determined from the detected misalignment.
In one implementation of the prior art pattern of FIG. 3, the pattern depicted in FIG. 3 is repeated multiple times with different colors. The outer lines 302 or inner lines 301 would be printed with a single color so as to permit one to gauge the printing misalignment from a reference color.